1. Field of the Invention
The present invention generally relates to optical reception apparatus and, more particularly, is directed to an optical reception apparatus for use with audio apparatus, such as a cordless speaker, a cordless microphone system or the like.
2. Description of the Prior Art
A known small and simple active speaker houses a battery-driven power amplifier within a speaker box and supplies an audio signal from a signal source to a speaker apparatus by means of a light. FIG. 1 shows in block form an example of a circuit arrangement of such speaker apparatus previously proposed by the assignee of the present application, and circuit elements and parts shown in FIG. 1 are all housed in the speaker box.
Referring to FIG. 1, shown at 1 is a light receiving device including a light receiving element, for example, a photodiode. The light receiving device 1 faces to the outside of the speaker box and receives an infrared light LT from a transmitter (not shown).
In this case, the infrared light LT is a signal light modulated by an audio signal SA. More specifically, the audio signal SA is converted into an FM (frequency-modulated) signal FT whose carrier frequency is 2.3 MHz and whose maximum frequency deviation falls within a range of .+-.150 kHz. The FM signal FT is supplied to an infrared LED (light emitting diode) together with a DC bias and the infrared LED derives the infrared light LT whose intensity (brightness) is modulated by the signal FT. The intensity of the infrared light LT becomes zero or substantially zero in the negative peak portion of the signal FT.
Further, the light receiving device 1 is connected with a coil provided at the input side of a bandpass filter 2 in a DC connection fashion, and the resultant DC circuit is connected through a de-coupling circuit 3 to a power supply battery 9.
The bandpass filter 2 is of .pi. type bandpass filter and has a pass band corresponding to the FM signal FT. Also, the battery 9 is formed of four dry batteries connected in series, i.e., 6 V. Accordingly, when receiving the infrared light LT from the transmitter (not shown), the light receiving device 1 derives the FM signal FT and supplied to the bandpass filter 2.
The FM signal FT from the bandpass filter 2 is supplied through a high frequency amplifier 4 to an FM receiving circuit 5. The FM receiving circuit 5 is formed of one-chip IC (integrated circuit) that is utilized by the standard FM receiver and includes circuit elements ranging from the high frequency amplifier to an FM demodulating circuit. Accordingly, in the FM receiving circuit 5, the signal FT is converted into an intermediate frequency signal having a frequency of 10.7 MHz and this intermediate frequency signal is FM-demodulated to provide the original audio signal SA. Then, the audio signal SA is supplied through a variable resistor 6 for adjusting a sound level and a power amplifier 7 to a speaker 8. A muting signal SM is supplied from the receiving circuit 5 to the power amplifier 7 so that, when the FM signal FT is not supplied to the receiving circuit 5, the power amplifier 7 is muted by the muting signal SM.
In order to control the power supply to the above circuits 4, 5 and 7, the following arrangement is required.
As shown in FIG. 1, an emitter-collector path of a power supply switch transistor 27 is connected in series between the battery 9 and the power supply line of the circuits 4, 5 and 7.
The signal FT from the bandpass filter 2 is supplied to a detecting circuit 10, and in this example, the detecting circuit 10 is formed of a narrow band AM receiving circuit. The voltage of the battery 9 is supplied to the detecting circuit 10 as an operation voltage without being switched by the power supply switch.
The signal FT from the bandpass filter 2 is supplied through a high frequency amplifier 11 to a .pi.-type tuning circuit 12 which is tuned with the signal FT. A negative impedance converting circuit 13 is connected to the output end of the tuning circuit 12 and an equivalent parallel resistance of the tuning circuit 12 is canceled by the negative input impedance indicated by the negative impedance converting circuit 13 so that the band width of the tuning circuit 12 falls within a range of 15 to 20 kHz.
Accordingly, the FM signal FT supplied to the tuning circuit 12 is slope-detected thereby and a detected signal DL is produced from the negative impedance converting circuit 13. Then, the detected signal DL is supplied through an amplifier 14 to an AM detecting circuit 15, from which there is derived a secondary harmonic signal HL of the audio signal SA. The secondary harmonic signal HL is supplied through a DC component reproducing circuit 21 to the base of a transistor 22.
Thus, when receiving the infrared signal LT, the optical reception device 1 derives the signal FT so that the transistor 22 is turned on by the signal HL. When not, the optical reception device 1 derives no signal FT and therefore the signal HL is not produced, thereby the transistor 22 being turned off.
When the infrared light LT is received by the optical reception device 1 and then the transistor 22 is turned on, whereby a transistor 24 is turned on and a transistor 25 is also turned on. When the transistor 25 is turned on, a transistor 26 is turned on and the transistor 27 is also turned on.
Accordingly, the voltage of the battery 9 is supplied through the transistor 27 to the circuits 4, 5 and 7, i.e., the power supply is turned on. Therefore, as described above, the amplifier 4 derives the FM signal FT, the receiving circuit 5 derives the audio signal SA and the audio signal SA is supplied through the amplifier 7 to the speaker 8. At that time, an LED 28 emits a light, indicating that the power supply is in its ON state.
However, when the transmitter (not shown) stops the transmission of the infrared light LT, then the infrared light LT is not received by the light receiving device 1 any more and the transistor 22 is turned off, whereby the transistor 24 is turned off and the transistor 25 is also turned off. When the transistor 25 is turned off, the transistor 26 is turned off and the transistor 27 is also turned off.
In that event, even when the transistor 22 is turned off, the transistor 25 is kept in its ON state during, for example, one minute by means of a time constant circuit 23 after the transistor 22 is turned off. Therefore, the power switch can be prevented from being turned off immediately after the transmission of the infrared light LT to the optical reception device 1 is temporarily interrupted by the obstacle or the like.
Further, at that time, since the FM signal FT is not supplied to the receiving circuit 5, the receiving circuit 5 derives a limiter noise. At that time, the amplifier 7 is muted by the muting signal SM, thereby preventing the limiter noise from being produced from the speaker 8.
As described above, according to the above speaker apparatus, a reproduced sound can emanate from the speaker without the power supply cord and the signal cord supplied thereto.
The above light receiving device 1 is generally constructed as shown in FIG. 2.
Referring to FIG. 2, it will be seen that a light receiving element 1C such as a photodiode or the like is shielded by a shielding member M of flat-box like configuration formed by molding a transparent plastic resin. Two connection terminals 1T, 1T are led out through the shielding member 1M in parallel to each other. In this case, of the planes of the shielding member 1M, the upper plane is utilized as a light receiving plane 1R for receiving the incident light (infrared light) LT, and the direction perpendicular to the light receiving plane 1R is selected as the front axis direction (.theta.=0 degree). Accordingly, the light receiving element 1C exhibits unidirectivity relative to the incident light LT as shown in FIG. 3. That is, the light receiving element 1C has sensitivity for the incident light LT whose incident angle lies in a range of substantially .vertline..theta..vertline..ltoreq.60.degree. and has no sensitivity substantially relative to the incident light LT which becomes incident from the lateral direction (.vertline..theta..vertline.=90.degree.).
Since the above-described speaker apparatus employs the light receiving device 1 shown in FIG. 2, this speaker apparatus produces a dead angle relative to the infrared light LT emitted from the transmitter, which unavoidably restricts the place where the speaker apparatus is installed.
To obviate this shortcoming, it is proposed to locate a lens 1L in front of the light receiving device 1 as shown in FIG. 4. This proposal, however, is not so effective because directivity is intensified to widen the dead angle although sensitivity in the front surface direction is increased as shown in FIG. 5.
Further, because the light receiving device 1 exhibits sensitivity relative to the front surface direction in a range of from .+-.60.degree., it is proposed to dispose three light receiving devices 1, 1 and 1, each having the same characteristic, in an angular displacement of 120 degrees each to thereby provide a non-directional light receiving device. In this case, however, the three light receiving devices 1, 1, 1 are needed, which unavoidably increases a manufacturing cost and which also degrades the space factor. Further, even when the three light receiving devices 1, 1, 1 are disposed as described above, the light receiving device is unavoidably arranged to have unidirectivity within the plane perpendicular to the sheet of drawing. As a consequence, if the light receiving device is arranged as a stereoscopic (or spherical) non-directional optical reception device, a larger number of light receiving devices must be combined, which cannot be realized in actual practice.